Lipoproteins are macromolecular complexes of lipids and proteins that are involved in the transport and distribution of dietary and/or endogenously synthesized lipids (e.g. cholesterol, triglycerides, and phospholipids) in the body. Plasma lipoproteins have traditionally been grouped in four major lipoprotein classes: Chylomicrons, very low density lipoproteins (VLDL), low-density lipoproteins (LDL), and high density lipoproteins (HDL).
The protein components of lipoproteins are called apolipoproteins and have been designated apoA-I, apoA-II, apoA-IV, apoB, apoCI, apoCII, apoCIII, and apoE. The biological roles played by each of these apolipoproteins in lipoprotein function and metabolism, and therefore in lipid transport and distribution, are being defined. Studies have revealed that genetic changes which alter either the function or the plasma concentration of any of these apolipoproteins can perturb the pathway of lipid metabolism and may directly or indirectly contribute to the pathogenesis of atherosclerosis and other disorders.
The plasma concentration of apolipoproteins may be altered by changes in the level of apolipoprotein gene transcription. Thus, it is important to understand the molecular mechanisms responsible for modulating apolipoprotein gene expression.
Gene transcription in eukaryotes is controlled by the interaction of nuclear proteins (transcription factors) with specific nucleotide sequences (regulatory elements, enhancers, silencers, etc.). Such interactions direct tissue-specific gene expression, gene expression during differentiation and development, and gene expression in response to intracellular and extracellular stimuli such as hormones and metabolites. Transcriptional regulatory elements are often found in front of (upstream) of genes, but can also be located within, or even downstream of, genes. In some cases, important transcriptional regulatory sequences that control tissue-specific or developmentally-regulated gene expression are located very far away, perhaps several kilobases, from the gene they regulate.
Studies of the transcriptional regulation of a gene (or genes) of interest often begin with DNA mapping analyses (e.g. DNase I protection studies or deletion analyses coupled with in vitro or in vivo transcription reactions) that identify broad sequence regions involved in modulating gene expression. However, identification of the exact protein factors and regulatory sequences involved, which is necessary for full understanding of the transcriptional regulation of a gene, is complicated by several realities of eukaryotic transcriptional regulation systems.
First of all, the regulatory elements recognized by different transcription factors are often adjacent or overlapping. Interactions between transcription factors bound to neighboring regulatory elements can profoundly alter the extent, or even the direction, of transcriptional regulation. In some cases, adjacent transcriptional activators can stimulate transcription synergistically, so that the increase in gene expression is greater than would be predicted by summing effects of the individual regulators. In other instances, proteins that are transcriptional activators in one regulatory context (i.e. in the presence of a particular set of neighboring regulatory factors) can function as transcriptional repressors under different circumstances (see, for example, Keleher et al. Mol. Cell. Biol. 9:5228-5230; Diamond et al. Science 249:1266-1272, 1990).
Secondly, individual regulatory sequences can often be recognized by more than one protein factor. In some cases, the different protein factors capable of binding to one regulatory element have opposite effects on gene expression. That is, both transcriptional activators and transcriptional repressors can sometimes recognize the same nucleotide sequence (see, for example Tanaka et al. Mol. Cell. Biol. 13:4531-4538, 1993).
Thirdly, some protein factors that are involved in regulating transcription from a particular promoter do not bind to regulatory elements within that promoter at all, but rather modulate gene expression by interacting with other factors that do bind to nucleotide sequence elements within that promoter.
For these and other reasons, identification and characterization of the exact regulatory elements and protein factors involved in transcriptional regulation is difficult. Such identification and characterization is required, however, for a full understanding of the transcriptional regulation of gene expression.